A typical OLED generates multicolor, intense light emission from vertical stacks of organic thin films sandwiched between charge injecting conductors. The OLED's organic materials are typically arranged in single or multilayer stacks. The conductors supply, upon the application of voltages with sufficient amplitude and polarity, both negative and positive charge carriers which recombine in the organic stack to release pure energy in the form of light. This phenomenon is called electroluminescence. In a vertically stacked OLED, at least one of the conductors must be optically transparent in order to couple light in the viewer's direction.
Typically, conductor materials for vertically stacked OLEDs are indium-tin-oxide (ITO) for the hole-injecting transparent conductor, and co-evaporated magnesium and silver (Mg:Ag) for the electron-injecting conductor. ITO is a problematic material for a number of reasons. First, ITO is normally deposited in a high-temperature sputtering process which can damage the organic material. Second, the deposition process for an ITO conductor is variable and can yield an unacceptably rough microstructure. Third, it is difficult to control the etching of the ITO layer in sub-micron dimensions using traditional wet etching techniques. Finally, the surface condition of ITO is unfavorable for injection and, as a result, oxidation is required to achieve high performance. Furthermore, in the traditional vertically stacked OLED architecture (e.g., ITO on one side with Mg:Ag on the opposite side of the organic material) the moisture-sensitive magnesium conductor has similar problems with patterning and environmental exposure.
In vertical OLEDs, the organic material is sequentially deposited in layers of molecular film. This sequential process is more time consuming and less efficient than the deposition of one layer of organic material comprised of a polymeric blend of the required material constituents. The fabrication process for the vertically stacked OLED also typically includes the need for mask changes during the sequential deposition of materials. The mask changes add further complexity and cost to the manufacturing process.
Vertically stacked prior art OLEDs, which employ a continuous (unpatterned) stack of organic materials, face the additional problem of unavoidable increases in lateral charge leakage. This charge leakage phenomenon typically occurs when one display device contains numerous OLEDs or pixel elements mounted on a substrate. The lateral charge conduction or leakage occurs along the substrate direction as the individual OLEDs decrease in size to submicron dimensions. These lateral leakage currents destroy the integrity of the overall display by creating crosstalk between the individual OLEDs or pixel elements.
Light piping is another problem associated with vertically stacked OLEDs. Light piping, a phenomenon also referred to as total internal reflection, results when light passes from a high dielectric constant material to a low dielectric constant material over a set of incidence angles determined by the laws of geometric optics and the frequency response of the materials. Light is confined to the high dielectric constant material. Light piping forms the basis for all fiber optic communications technologies. Light piping in the glass substrate creates optical losses. Both light piping and current leakage reduce the overall external efficiency of the display device. Accordingly, there is a need for microcavity structure within the OLED to provide output enhancement.
The typical applications of OLEDs include, for example, flat panel displays, optical interconnects, optical fiber communications and LED printing. Given the nature of these fields, the need exists for an OLED which is reliable and capable of withstanding forces associated with its use. The vertically stacked OLED does not provide any inherent structural stability. The horizontal layers are merely stacked upon one another without any means for resisting lateral or forces. Accordingly, there is a need for an OLED structure which provides increased support and stability for the organic layers.
The present invention addresses the above problems, in whole or in part, via a display device, which utilizes transversely stacked OLEDs. Each OLED of the present invention typically serves as one color component of a color pixel. The present invention also addresses the problems associated with patterning a multicolor display device due to the degradation or destruction of organic light-emitting materials through exposure to moisture, oxygen, light, temperature and chemicals. This exposure unavoidably results from traditional semiconductor fabrication methods. The present invention solves the aforementioned problems with traditional OLEDs, and provides other benefits as well.